In recent years, people have an increasingly distinct demand for a high bandwidth, and their demand for a high bandwidth also has been fueled by the constant development of various value-added services, e.g., applications of video conferences, high-definition images, real-time games, IPTV, etc.
In general, modern communication networks can be divided into a core network and an access network, and the access network is referred figuratively to as “the last mile”. Among numerous solutions for the existing market, the emergence of “Fiber To The Home (FTTH)” is considered as an ultimate solution to “the last mile” of a broadband access. Among numerous FTTH solutions, a Passive Optical Network (PON) has gained popular attention as a predominant optical access approach at present. In recent years, the PON technology has evolved from an Asynchronous Transfer Mode Passive Optical Network (APON) to a Broadband Passive Optical Network (BPON) and further to an existing Ethernet Passive Optical Network (EPON) and Gigabit-Capable Optical Network (GPON), and people pay increasing attention to the PON technology with a bandwidth above 1 Gbit/s, i.e., the EPON and the GPON, along with the popularization of an Ethernet technology in a Metropolitan Area Network (MAN) and the development of a broadband access technology.
The GPON has the maximum transmission rate up to 2.5 Gbps and supports a variety of services including Asynchronous Transfer Mode (ATM), Ethernet, Time Division Multiple (TDM), Community Antenna Television (CATV), etc., and it can be said the PON network technology which is the most functionally perfect at present is also a very economic and broadband oriented network access approach.
The GPON can better accommodate a broadband network access application due to its crucial advantages of, e.g., a higher rate, a higher access performance and network efficiency, strong flexibility and scalability, etc., which are particularly as follows:
1. High rate: the GPON can offer a wider range of data rates than the other passive optical network technologies, support a variety of symmetric and asymmetric line rates as well as the maximum rates up to 2.488 Gbps (for a downlink data stream) and 1.244 Gbps (for an uplink data stream) and satisfy a bandwidth demand of various existing and further possible services.
2. High efficiency: the GPON with an efficiency of 100% at a rate of 1.25 Gbps is apparently more attractive than the EPON with an efficiency of only 50%, and the GPON solution can provide a user with a higher bandwidth.
3. Strong scalability: the GPON can bear various types of user data in the protocols of both ATM and GFP. Both uplink and downlink frames thereof have a length of 125 μs. Broadcast is adopted in the downlink, and a Time Division Multiple Access (TDMA) approach is adopted in the uplink. The uplink frame is composed of multiplexed burst transmission timeslots and each includes transmission timeslots of one or more Optical Network Units (ONUs), and transmission of uplink data of the respective ONUs is indicated in an uplink bandwidth mapping field of the downlink frame, so that the GPON can bear an increasing number of various protocols and technologies, including a support of numerous TDM and data services and a support of new applications of, e.g., a storage area network, data video, etc.
A GPON system is composed of an Optical Line Termination (OLT), an Optical Distribution Network (ODN) and an Optical Network Unit/Optical Network Termination (ONT) in the same way as other PON systems. One of crucial technologies is to design a GPON OLT module for a GPON network to be deployed.
In the GPON system, it is of great importance for the Optical Line Termination (OLT) module to perform accurate and rapid reception of burst data in a burst data packet transmitted in the uplink. Since a stringent criterion was established in the GPON protocol and the dominant telecommunication operators proposed at the end of 2008 that a digital Received Signal Strength Indicator (RSSI) function shall be added to the GPON OLT module, thus resulting in a significantly increased difficulty of designing the GPON OLT module.